Đang chuẩn bị liên kết để tải về tài liệu:
Structural Steel Designers Handbook Part 15

Tham khảo tài liệu 'structural steel designers handbook part 15', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 15.78 SECTION FIFTEEN cables and pylons support the girder. When these redundants are set equal to zero an articulated statically determinate base system is obtained Fig. 15.56b. When the loads are applied to this choice of base system the stresses in the cables do not differ greatly from their final values so the cables may be dimensioned in a preliminary way. Other approaches are also possible. One is to use the continuous girder itself as a statically indeterminate base system with the cable forces as redundants. But computation is generally increased. A third method involves imposition of hinges for example at a and b Fig. 15.57 so placed as to form two coupled symmetrical base systems each statically indeterminate to the fourth degree. The influence lines for the four indeterminate cable forces of each partial base system are at the same time also the influence lines of the cable forces in the real system. The two redundant moments Xa and Xb are treated as symmetrical and antisymmetrical group loads Y Xa Xb and Z Xa - Xb to calculate influence lines for the 10-degree indeterminate structure shown. Kern moments are plotted to determine maximum effects of combined bending and axial forces. A similar concept is illustrated in Fig. 15.58 which shows the application of independent symmetric and antisymmetric group stress relationships to simplify calculations for an 8-degree indeterminate system. Thus the first redundant group X1 is the self-stressing of the lowest cables in tension to produce M1 1 at supports. The above procedures also apply to influence-line determinations. Typical influence lines for two bridge types are shown in Fig. 15.59. These demonstrate that the fixed cables have a favorable effect on the girders but induce sizable bending moments in the pylons as well as differential forces on the saddle bearings. Note also that the radiating system in Fig. 15.55c and d generally has more favorable bending moments for long spans than does the harp system